The invention relates to methods for access to a medium by a multi-channel device. The medium comprises a transmission system having at least two channels on which a message to be transmitted comprises at least a preamble and a header plus a succeeding data field. The data field may contain either user data, which is termed “useful data”, or control information, for coordinating the access to the medium for example. A method for decentralized medium access and a method for centralized medium access are claimed.
The frequency band of a transmission system is often divided into sub-bands, which are termed channels, on which either a single communications link or else a complete cell of a system operates. The latter is the case with for example 802.11a/e and HiperLAN/2 wireless local area networks (WLANs). With regard to 802.11 a/e WLANs, the basic specification, namely ANSI/IEEE Std 802.11, 1999 edition, and “IEEE Std 801.11a-1999: High-speed Physical Layer in the 5 GHz Band, Supplement to Standard IEEE 802.11”, IEEE New York, September 1999 and “IEEE Std 802.11e/D4.2: Medium Access Control (MAC) Enhancements for Quality of Service (QoS), Draft Supplement to Standard IEEE 802.11”, IEEE New York, February 2003, are hereby incorporated by reference.
The bandwidth of the channel sets a limit to the maximum data rate that can be obtained between two stations in a cell, or in other words to the capacity of a cell. One possible way of increasing the capacity of a transmission system is to enlarge the bandwidth of a communications channel. With a preset channel definition, this can be achieved by grouping two or more channels to obtain one channel of greater width. This approach is known in theory and has been implemented in certain WLAN systems, in which a high data rate is obtained or a what is called “turbo” mode of handling.
There are a certain number of stations in a transmission system, and of these two or more at a time make a temporary connection with one another. The grouping of channels can only be performed if all the stations involved operate in the mode where the data rate is high. For channel grouping, there has to be one standard for all the stations or terminals. However, mobile telecommunications is the very area in which there are a variety of manufacturers of terminals and the terminals employ channel grouping that is not standardized. Because channel grouping, as a performance feature, entails additional expenditure in development and manufacture, it may also be the case that, because of the cost, a manufacturer of terminals offers on the one hand terminals employing channel grouping and on the other hand terminals that do not have this performance feature. Also, older terminals that were developed and sold before channel grouping was introduced in transmission systems are incapable of operating in the mode having the high data transmission rate.
Standard IEEE 802.11e combines both a decentralized and a centralized scheme for medium access. An algorithm that can be used for this will be explained by taking the 802.11 system as an example. The basic 802.11 MAC protocol is the distributed coordination function (DCF), that operates as a listen-before-talk scheme. The distributed coordination function is based on carrier multiple sense access (CMSA). Having found that no other transmissions are underway on the wireless medium, the stations emit MAC service data units (MSDUs) of an arbitrary length of up to 2304 bytes. If, however, two stations find that a channel is free at the same time, a collision occurs when the data is sent out over the radio-transmitting medium, namely air, that they both use. The 802.11 MAC protocol defines a mechanism for collision avoidance (CA) to reduce the probability of collisions of this kind. It is part of the collision avoidance mechanism that, before it starts transmitting, a station performs a waiting or backoff process. The station continues listening out on the channel for an additional, random period of time after it finds that the channel is free. Only if the channel remains free for this additional random length of time is the station permitted to initiate a transmission. This random waiting time is composed of a constant portion, the what is called DCF interframe space (DIFS), which is 34 μs long in the case of the 802.11a MAC protocol, and a random portion whose length is between zero and a maximum time. The DIFS space is thus the minimum possible waiting time for the stations. The length of the random portion of the waiting time is obtained as a multiple of the length of a time slot (slot time), which length is 9 μs in the 802.11a MAC protocol. Each station draws a random value, for the number of slot times to be waited, which it stores in the what is called contention window (CW). On the expiry of each period of 9 μs, the value of the CW is decremented by 1.
Each time a data frame is successfully received, the receiving station immediately sends out an acknowledgement frame (ACK). The size of the contention window is enlarged if a transmission fails, which means that a data frame sent out has not been acknowledged. After each failed attempt at a transmission, a new medium access is effected after a fresh waiting time, with the fresh waiting time being selected to be twice as long as the current contention window. This reduces the likelihood of a collision in the event that a plurality of stations are trying to gain access to the channel. Those stations that deferred channel access during the time when the channel was busy do not select a new random waiting time but continue the countdown of the time for the deferred medium access on finding that the channel is idle again. In this way, stations that deferred channel access due to their longer random waiting time as compared with other stations are given a higher priority when they resume their efforts to start a transmission. After each successful transmission, the transmitting station performs a new random waiting process (backoff) even if it does not have a further MSDU to send at the time. This is what is called the “post backoff”, because this waiting process takes place after rather than before a transmission.
There is a situation under the 802.11 MAC protocol in which a station does not have to perform a waiting process of random duration (a backoff) before it can start transmitting data. This situation arises if an MSDU from a higher layer arrives at a station and the post-backoff for the last transmission has already been completed, or in other words, there is no queue and, in addition, the channel has been idle for a minimum DIFS time. All subsequent MSDUs that arrive after this MSDU will be transmitted after a random waiting time until there is again no queue.
To limit the probability of long frames colliding and being transmitted more than once, data frames are also fragmented. A long MSDU can be divided by fragmentation into a plurality of small data frames, i.e. fragments, that can be transmitted sequentially as data frames to be acknowledged individually. The advantage of fragmentation is that, if a transmission fails, this failure can be detected at an earlier point in time and hence less data has to be re-transmitted.
In systems using CSMA, there is a problem with hidden stations. This is a problem inherent in the CSMA system and to alleviate it the system defines a request-to-send/clear-to-send (RTS/CTS) mechanism that can be used as an option. Before data frames are transmitted, it is possible for a system to send a short RTS frame, which is followed by a CTS transmission from the receiving station. The RTS and CTS frames contain information on the length of the transmission time of the next data frame, i.e. the first fragment, and of the corresponding ACK response. What is achieved in this way is that other stations near the transmitting station, and hidden stations near the receiving station, do not start a transmission, because they set a counter, the what is called Network Allocation Vector (NAV). The RTS/CTS mechanism helps to protect long data frames against hidden stations. With fragmentation, a large number of ACKs are transmitted, whereas with RTS/CTS the MSDU can be transmitted efficiently in a single data frame. Between each successive pair of frames in the sequence RTS frame, CTS frame, data frame and ACK frame, there is a short interframe space (SIFS), which is 16 μs long under 802.11a.
FIG. 1, which relates to the prior art, is a diagram showing an example of a distributed coordination function (DCF). A short interframe space (SIFS) is shorter than a DCF interframe space (DIFS), as a result of which CTS responses and acknowledgement frames (ACKs) always have the highest priority for access to the wireless medium. In the latest version of the MAC protocol, the 802.11e protocol, an Enhanced Distribution Coordination Function (EDCF) has been introduced, which still operates in the same way but, in addition, supports different types of traffic, such as, for example, access priorities. In the time-based diagram for six stations shown in FIG. 1, although station 6 cannot detect the RTS frame of the station 2 that is transmitting, it can detect the CTS frame of station 1.
Another known function, the Hybrid Coordination Function (HCF) extends the rules for (E)DCF access. A crucial performance feature of 802.11e MAC is the Transmission Opportunity (TXOP). A TXOP is defined as the interval between the point at which a station receives the right to initiate a transmission, defined by the starting time, and a maximum duration. TXOPs are allocated by way of contentions (EDCF-TXOP) or are granted by HCF (polled TXOP). Only one station in the cell, which is called the hybrid coordinator (HC), can give other stations permission to transmit, i.e. can grant a TXOP. The duration of a polled TXOP is specified by the time field within the allocating frame. The hybrid coordinator is able to allocate TXOPs to itself, to enable MSDU transmissions to be initiated, at any time, but only on detecting that the channel is idle for a time equal to a PIFS (Point Coordinator Interframe Space), which time is shorter than the length of a DIFS.
Defined as part of the 802.11e protocol is an additional random access protocol that enables collisions to be reduced. What is called Controlled Contention gives the hybrid coordinator an opportunity of learning what stations need to be queried at what times with regard to their wishes to transmit. The controlled contention mechanism allows stations to request to be allocated polled TXOPs by sending a source query, without interfering with other (E)DCF traffic.